Newer Tost hydraulic brake issue

[[email protected]]

The Tost hydraulic brake cylinder mounted to my stick failed on my last flight. I've emailed Tost, but so far no response, which certainly might be virus related and therefore understandable.

The brake worked fine before takeoff but before landing there was zero resistance when squeezing the lever. I'm 99.9% sure its NOT an issue with air in the system and just to check I flushed from bottom to top and no change and no bubbles exposed.

I suspect that if there is a check valve inside the piston of the cylinder that it has failed open. There is a swishing sound when the lever is squeezed as if fluid is being pushed through an orifice (no external leaks found) so this matches my hypothesis.

If you have actual knowledge of the internal design of this hydraulic cylinder, particularly if you've taken one apart, please message me directly.

Thanks,

Mark

[2G]

On Tuesday, April 7, 2020 at 5:29:59 PM UTC-7, wrote:
> The Tost hydraulic brake cylinder mounted to my stick failed on my last flight. I've emailed Tost, but so far no response, which certainly might be virus related and therefore understandable.
>
> The brake worked fine before takeoff but before landing there was zero resistance when squeezing the lever. I'm 99.9% sure its NOT an issue with air in the system and just to check I flushed from bottom to top and no change and no bubbles exposed.
>
> I suspect that if there is a check valve inside the piston of the cylinder that it has failed open. There is a swishing sound when the lever is squeezed as if fluid is being pushed through an orifice (no external leaks found) so this matches my hypothesis.
>
> If you have actual knowledge of the internal design of this hydraulic cylinder, particularly if you've taken one apart, please message me directly.
>
> Thanks,
>
> Mark

This should be obvious, but firstly check your brake pads to be sure they are not worn. Replace if they are.

Second, inspect all fittings and lines for any evidence of a leak. This happened to me on my ASH2e; inspecting the brake line between the reservoir and the brake shoe required pulling the engine.

Third, completely bleed the brake fluid from the brake and measure how much you get. This video may help:
https://www.youtube.com/watch?v=qOtrnsNo8K0

Forth, you will need to put new brake fluid in the lines. Obviously, you will need the exact type of fluid specified by Tost - do not use any other brake fluid, certainly not automotive brake fluid. You will need a way of pressurizing the fluid; Schleicher recommends a stand similar to water ballast filling, but I think what the guy in this video used is better:
https://www.youtube.com/watch?v=bZlq87VXybU
Depending upon where your reservoir is located, you may need an assistant to tell you when to stop filling.

There is no certain way to know that there is no air bubbles in your brake line, so you must go thru this process.

Good luck!

Tom

[[email protected]]

Just a quick followup. I managed to get connected with Tost and they have been super helpful, going above what I expected to help me resolve my issue.
Thanks to those who responded.

Mark

[Stephen Szikora]

You’re leaving us hanging ... what was the issue?

[Charlie M. (UH & 002 owner/pilot)]

Agreed.....Frikkin fill us in....saves peeps pounding on "support" for similar issue....

[[email protected]]

Tost went put of their way to send me some stuff before the Easter holiday, but at the moment those parts have not arrived (I suspect sometime next week) and then I’ll get back out to the airport and see what I can discern when I start digging in. At the moment I still don’t know what is the underlying issue. Rain forecast for the weekend so flying is moot, brake or no brake....

[[email protected]]

Removed the master cylinder assy today and brought it home. Not the easiest to take apart, but when I did there wasn’t a clear failure to point to. There is a check valve in the design and I still suspect that was hung open. The primary cup seal looked fine. There was no deep scoring of the inside cylinder wall. A little wear was evident, but not likely an issue. There was a very slight indentation on the small o-ring that seals the check valve, but even that didn’t look like it was worthy of mention. Maybe a small piece of dirt got in somewhere that wasn’t obvious. I put it all back together and figure I’ll get some more Dot 4 and see what happens in a small bench test at some point. Overall a nicely designed unit so kinda odd that it failed.

[Charlie M. (UH & 002 owner/pilot)]

Double check fluid type....while I do DOT-4 on many things (cross compatible with older DOT types), some aircraft brake systems use something different and they DON'T cross!!!!